How your pregnancy timeline is calculated
Everything here is anchored to the first day of your last menstrual period (LMP). The estimated due date is the LMP plus 280 days (40 weeks), the standard convention known as Naegele's rule. The estimated conception date is placed about two weeks after the LMP, since ovulation and fertilisation typically happen near the middle of a cycle.
Gestational age is simply the time elapsed between the LMP and the "as of" date you choose, expressed as completed weeks plus extra days. Counting from the LMP rather than from conception is deliberate: the period is an easy landmark to record, whereas the exact moment of conception is almost never known.
Reading gestational age and trimesters
A figure like "12 weeks 3 days" means twelve full weeks have passed since the LMP, plus three days into the thirteenth week — the same shorthand clinicians use. The trimester is derived from that age.
The first trimester covers the LMP through week 12, the second runs from week 13 through week 27, and the third from week 28 to birth. Each stage brings different milestones, screenings, and physical changes.
How accurate is it, and what changes the estimate
Because pregnancy is dated from the LMP, the figures assume ovulation around day 14 of a regular cycle. If your cycle is longer, shorter, or irregular, the true gestational age can differ from the estimate.
- Only about 1 in 20 babies arrive on the exact due date.
- A delivery from roughly 37 to 42 weeks is considered full term.
- An early ultrasound dating scan is more precise and may revise the due date.
- An uncertain or unusually timed LMP reduces the reliability of every figure shown.
Medical disclaimer
This calculator is for general information and education only and does not replace professional prenatal care. It cannot confirm pregnancy or assess the health of a pregnancy. Always rely on the assessment of a doctor or qualified healthcare provider for dating and monitoring your pregnancy.
Formula
dueDate = LMP + 280 days; conception ≈ LMP + 14 days; gestational age = (asOfDate − LMP)Frequently asked questions
- Why is pregnancy dated from the last period?
- The LMP is an easy, reliable landmark to record, while the exact date of conception is rarely known. Convention adds 280 days to the LMP to estimate the due date.
- How accurate is the due date?
- It is an estimate. A normal delivery can occur anywhere from about 37 to 42 weeks. An ultrasound dating scan may adjust the estimate.